Electrically programmable fuse

ABSTRACT

In this invention is described an electrically programmable fuse that uses a floating gate to control the fuse action. The activation of the fuse can be done at any time during the life of the product containing the fuse. By programming a charge onto the floating gate an active transistor is made to conduct or not to conduct. The fuse can be reused by re-programming the fuse to the previous state. Different states of the fuse which represent product options can be obtained by programming appropriate voltage levels.

This is a division of patent application Ser. No. 09/080,115, filingdate May 18, 1998, now U.S. Pat. No. 6,100,746, An ElectricallyProgrammable Fuse, assigned to the same assignee as the presentinvention.

BACKGROUND OF THE INVENTION

1. Field of Invention

This invention is related to integrated circuits and more particularlyelectrically programmable fuse devices.

2. Description of Related Art

A fuse device used on an integrated circuit is for the purpose ofrepairing a defect or selecting a functional option. The operation ofusing a fuse has required the use of a laser to vaporize a thin piece ofpoly-silicon that made up the fuse. This requires the fuse to be openedby the laser before final packaging of the semiconductor, and once thefuse is opened there is no recovery to the original state. With thislimitation there has been a push to find ways to avoid the limitationsof a fuse requiring a laser to break the connection.

In U.S. Pat. No. 5,642,316 (Tran et al.) a source follower EEPROM memoryfuse is used to program memory redundancy circuits for repair ofdefective memory rows. The redundant memory circuits are initiallyoutside the normal memory address range and through the use of fuses arebrought into the normal memory address range to replace defective memorybits . This EEPROM fuse can be reused many times. In U.S. Pat. No.5,258,947 (Sourgen) EEPROM cells are used in a regular memory state andcan be programmed to perform the function of a fuse. The fuse functionresults from the breakdown of the tunnel oxide and places the memorycell into an irreversible state. In U.S. Pat. No. 5,233,566 (Imamiya etal.) a floating-gate, avalanche injected, MOS transistor (FAMOS) memorycell is connected in series with a fuse to provide a redundant memorycell. In U.S. Pat. No. 4,852,044 (Turner et al.) is described a securityfuse device for a programmed logic device (PLD) that uses charge storedon a floating gate of a transistor to prevent access to the PLDarchitectural data.

In reference to “Silicon Processing for the VLSI Era” by Stanley WolfPh.D., Lattice Press 1990, pp 624-625, a floating-gateavalanche-injection MOS transistor (FAMOS) device is described in whichcharge is injected into a gate from hot electrons produced by avalanchebreakdown of the drain-substrate pn junction. Once the electrons aretransferred to the gate they are trapped there because of the potentialenergy barrier at the oxide-silicon interface.

A fuse like device that can be programmed to be on or off afterpackaging and hold its switching state for years can be a very usefultool. Not only can it be used for the classical repair of memories butit can also be used to activate or deactivate function and features of aparticular circuit. These functions and features being changed at thecommand of the user provides additional flexibility without requiringfactory intervention. Also being able to reverse a decision to chose afunction can be very valuable particularly when problems arise.

SUMMARY OF THE INVENTION

This invention is an electronic rise like device that is made up of twosemiconductor devices connected by a floating gate. In a firstembodiment a first of the two transistors is a P-channel device seatedin an N-well on a P-substrate that provides the activation andde-activation of the electronic fuse. A second transistor connected tothe floating gate is an N-channel transistor which is on or offdepending upon the charge on the floating gate and provides a fuse likefunction.

Charge is programmed onto the floating gate by means of hot electrons orFowler-Nordheim tunneling in the first transistor to enable the secondtransistor to be on and act as a non-blown fuse. Charge is remove fromthe floating gate using Fowler-Nordheim tunneling, turning off thesecond transistor, and enabling the second transistor to act as an openfuse.

In a second embodiment of this invention, a heavily doped P+ region isimplanted around one end of a floating gate in an N well residing on a Psubstrate. At the other end of the floating gate is an N-channeltransistor in the P-substrate. The heavily doped P+ region provides themeans for programming charge onto the floating gate using eitherFowler-Nordheim tunneling or hot electrons, and the N-channel transistoris either on or off depending upon the charge on the floating gate.Charge is removed from the floating gate using Fowler-Nordheimtunneling.

In a variation of the second embodiment of this invention an N+ deviceformed by a heavily doped N+ region is implanted around one end of afloating gate in a P well residing in an N well on a P substrate. Theother end of the floating gate is an N-channel transistor in the P wellresiding in an N well on the P substrate. The N+ device provides themeans for programming charge onto the floating gate using either hotelectrons or Fowler-Nordheim tunneling, and the N-channel transistor iseither on or off depending upon the charge on the floating gate. Chargeis removed from the floating gate using Fowler-Nordheim tunneling.

BRIEF DESCRIPTION OF THE DRAWINGS

This invention will be described with reference to the accompanyingdrawings, wherein:

FIG. 1a is a plan view of the electrically programmable fuse device;

FIG. 1b is the vertical structure of the electrically programmable fusedevice;

FIG. 2a is a plan view of a second embodiment of the electricallyprogrammable fuse device;

FIG. 2b is the vertical stricture of a second embodiment of tieelectrically programmable fuse device;

FIG. 2c is the vertical structure of a variation of the secondembodiment of the electrically programmable fuse device;

FIG. 3a shows a schematic of a possible usage of the fuse device;

FIG. 3b shows an additional schematic of a possible usage of the fusedevice; and

FIG. 4 is a diagram of device threshold, Vt, versus time the fuse isbeing programmed.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

Referring to FIG. 1a, a plan view of the electrically programmable fuseis shown. For convenience of illustration and relationship to the crosssection in FIG. 1b, a floating gate 11 connecting an N-channel and aP-channel device is shown as “U” shaped; however, the floating gate canbe any shape that is commensurate with the layout requirements. Thefloating gate 11 connects two transistors, a P-channel transistor with adrain 12 and source 13 and an N-channel transistor with drain 16 andsource 15. A cross section view AA is shown in FIG. 1b. The P-channeltransistor is constructed of P+ diffusions for a drain 12 and source 13which reside in an N-well 14. The N-channel transistor on the oppositeend of the floating gate 11 and has an N+ source 16 and an N+ drain 15that reside in the P substrate 18.

Continuing to refer to FIG. 1b, a gate control voltage V_(GC) 22,connected to the N well 14, the P+ drain 12 and the P+ source 13,programs the electrically programmable fuse. A voltage Vs 20 isconnected to the source 16 of the N-channel transistor and a voltageV_(D) 21 is connected to the drain 15 N-channel transistor. A voltageV_(B) 23 is connected to the semiconductor substrate 18. Programmingcharge onto the floating gate 11 is done by means of hot electrons orFowler- Nordheim tunneling when a high positive voltage is applied toV_(GC) 22. For example, when programming the fuse to be on (conducting)using hot electrons, V_(GC)=approximately +6 to +7 volts, V_(B)=V_(S)=0volts, and V_(D)=approximately +5 to +7 volts. When programming theelectronic fuse to be on using the Fowler-Nordheim tunneling mechanism,V_(GC)=approximately +8 to +10 volts, V_(D)=V_(S)=+3.5 volts and V_(B)=0volts. In order to erase the charge on the floating gate 11 and turn off(non-conducting) the electrically programmable fuse, Fowler-Nordheimtunneling is used with a high negative voltage being applied to V_(GC)22, for example V_(GC)=approximately −6 to −10 volts, V_(S)=+5 volts andV_(D) is floating. To read the N-channel transistor and use it as a fusewhich is blown or not blown, the following circuit voltages need to beapplied, V_(GC)=+5 volts, V_(D)=+1.5 volts and V_(S)=V_(B)=0 volts.

In FIG. 2a is shown a plan view of a second embodiment of theelectrically programmable switch. A “U” shaped floating gate 11 is usedfor illustrative purposes and can be any shape commensurate with thelayout requirements. At one end of the floating gate 11 is P+ diffusion17 and at the other end of the floating gate is an N-channel transistorhaving a drain 16 and a source 15. A cross section view BB is shown inFIG. 2b where the P+ diffusion 17 is shown within an N-well 14, and theN+ drain 16 and the N+ source 15 of the N-channel transistor are shownresiding in the P substrate 18.

Continuing to refer to FIG. 2b, a gate control voltage V_(GC) 22 isconnected to the P+ diffusion 17 and the N well. Connected to the source16 of the N-channel transistor is V_(S) 20 and connected to the drain 15is the voltage V_(D) 21. A voltage V_(B) 23 is connected to thesemiconductor substrate 18. Programming of the charge on the floatinggate 11 is done by means of hot electrons or Fowler-Nordheim tunnelingwhen a high positive voltage is applied to V_(GC) 22. For example, whenprogramming the fuse to be on (conducting) using hot electrons,V_(GC)=approximately +6 to +7 volts, V_(B)=V_(S)=0 volts, andVD=approximately +5 to +7 volts; and when programming the electronicfuse to be on using the Fowler-Nordheim tunneling mechanism,V_(GC)=approximately +8 to +10 volts, V_(D)=V_(S)=+3.5 volts and V_(B)=0volts. In order to erase the charge on the floating gate 11 and turn off(non-conducting) the electrically programmable fuse, Fowler-Nordheimtunneling is used with a high negative voltage being applied to V_(GC)22, for example V_(GC)=approximately −6 to −10 volts, V_(S)=+5 volts andV_(D) is floating. To read the N-channel transistor and use it as a fusewhich is blown or not blown, the following circuit voltages need to beapplied, V_(GC)=+5 volts, V_(D)=+1.5 volts and V_(S)=V_(B)=0 volts.

In FIG. 2c is a cross section of a variation of the second embodiment ofthis invention. An N+ diffusion 17 is located at one end of a floatinggate 11 in a P well 14. The other end of the floating gate forms atransistor with an N+ source 16 and an N+ drain 21 and residing in the Pwell 14. The P well 14 resides inside an N well 19 on a P substrate 18.A voltage V_(GC) 22 connects to the P well 14 and the N+ diffusion 17,and a voltage V_(B) 23 is connected to the semiconductor substrate 18.Connected to the source 16 of the N-channel transistor is a voltageV_(S) 20, and connected to the drain 15 is a voltage V_(D) 21.

Continuing to refer to FIG. 2c, programming of the charge on thefloating gate 11 is done by means of hot electrons or Fowler-Nordheimtunneling when a high positive voltage is applied to V_(GC) 22. Forexample, when programming the fuse to be on (conducting) using hotelectrons, V_(GC)=approximately +6 to +7 volts, V_(B)=V_(S)=0 volts, andVD=approximately +5 to +7 volts; and when programming the electronicfuse to be on using the Fowler-Nordheim tunneling mechanism,V_(GC)=approximately +8 to +10 volts, V_(D)=V_(S)=+3.5 volts and V_(B)=0volts. In order to erase the charge on the floating gate 11 and turn off(non-conducting) the electrically programmable fuse, Fowler-Nordheimtunneling is used with a high negative voltage being applied to V_(S) 20and V_(D) 21, for example V_(GC)=0 volts and V_(S)=V_(D)=approximately+8 to +10 volts. To read the N-channel transistor and use it as a fusewhich is blown or not blown, the following circuit voltages need to beapplied, V_(GC)=+5 volts, V_(D)=+1.5 volts and V_(S)=V_(B)=0 volts.

Shown in FIG. 3a is a possible circuit configuration for theelectrically programmable fuse 30. The N-channel transistor 31 of thefuse 30 is connected between ground 37 and other circuitry representedby a P-channel transistor 36 connected to V_(DD). The gate 32 of the Nchannel transistor 31 of the fuse 30 is a floating gate connected to thegate of the P-channel transistor 33 which is also floating. The N-wellof the P-channel transistor 34 is connected to (a electrical terminal 35to provide capability to set the fuse 30 into different modes whichinclude “on”, “off” and higher threshold voltages. The source and drainof the P-channel transistor 33 can also be used to program the state ofthe fuse and can be connected to other circuitry to turn the fuse on andoff.

In FIG. 3b is shown a possible connection of the electricallyprogrammable fuse 30 with an N-channel transistor 40 representing othercircuitry connected to ground 37. The N-channel transistor 31 of theelectrically programmable fuse 30 is connected to a P+ device 33 bymeans of the floating gate 32. The N-well of die P-channel transistor 34is connected to an electrical terminal 35 to provide capability to setthe fuse 30 into different modes which include “on”, “off” and higherthreshold voltages. The source and drain of tie P-channel transistor 33can also be used to program the state of the fuse and can be connectedto other circuitry to turn the fuse on and off.

In FIG. 4 is shown the relationship between the level of the thresholdvoltage Vt and lie time (logarithmic scale) required during programmingof charge on the floating gate. Depending upon the user's requirementsdifferent voltages can be used to set the state of the fuse. Forexample, Vt=2.0 volts for operation in a 2.5 volt circuit, Vt=4 voltsfor operation in a 3 volt circuit and Vt=6 volts for operation in a 5volt circuit.

While the invention has been particularly shown and described withreference to preferred embodiments thereof, it will be understood bythose skilled in the art that various changes in form and details may bemade without departing from the spirit and scope of the invention.

What is claimed is:
 1. A programmable electronic fuse, comprising: a) afloating gate connecting a transistor to a semiconductor device, b) saidsemiconductor device comprising a P+ diffusion in an N well, c) saidtransistor an N-channel device, d) said transistor being on or offdepending upon a charge on the floating gate, e) said fuse programmableby changing the charge on the floating gate induced by a voltage appliedto said N well and said P+ diffusion.
 2. The programmable electronicfuse of claim 1, wherein the P+ diffusion surrounds one end of thefloating gate.
 3. The programmable electronic fuse of claim 1, whereinthe fuse is an electronic switch that is controlled to be open andclosed by the charge induced on said floating gate.
 4. The programmableelectronic fuse of claim 1, wherein the fuse has an on mode and off modewherein said fuse can be switched from said on mode to said off mode andfrom said off mode to said on mode.
 5. The programmable electronic fuseof claim 1, wherein the fuse is turned off when both the P+ diffusionand the N-well located at one end of the floating gate are connected toV_(GC) that is equal to approximately around −6 to −10 volts.